The present invention relates to semiconductor devices such as an insulated-gate bipolar transistor (IGBT), and more particularly to its low-loss implementation.
The insulated-gate bipolar transistor (IGBT) is a switching element in which a current flown between a collector electrode and an emitter electrode is controlled using a voltage applied to a gate electrode. This IGBT is provided with features that it is capable of dealing with a comparatively wide range of power, and that its switching frequency is broad enough. Accordingly, in recent years, the IGBT has been in wide use, which ranges from home-use small-power appliances, such as air conditioners and microwave ovens, to large-power appliances such as inverters in railroads and steelmaking plants.
Among these IGBT's performances, one of performances whose improvement is requested most seriously is a reduction in its loss. In recent years, implementation of low-loss IGBTs have been considered and designed.
For example, FIG. 11 illustrates a planar-type highly-conductive IGBT disclosed in JP-A-10-178174. In this IGBT, a p layer 100 is in contact with a collector electrode C. Moreover, an n layer 111, whose carrier concentration is lower than that of this p layer 100, is multi-layered on the p layer 100. An n− layer 110, which has a substantially uniform carrier concentration lower than that of the n layer 111, is multi-layered on the n layer 111. An n layer 150 is diffused on the other surface side of this n− layer 110. A p layer 120 is formed within the n layer 150, and further, an n+ layer 130 is formed within this p layer 120. On the surfaces of the n+ layer 130, the p layer 120, the n layer 150, and the n− layer 110, a MOS gate is provided which is formed by including an insulating film 300, an insulating film 400, and a gate electrode G insulated with these insulating films 300 and 400.
Meanwhile, a p+ layer 121 is formed on the surface of the p layer 120. The p+ layer 121 and the n+ layer 130 are in low-resistance contact with an emitter electrode E. The respective electrodes E, C, and G are electrically guided to terminals which correspond thereto respectively.
In this IGBT, its main feature is that the n layer 150 is formed on the periphery and circumference of the p layer 120. By providing with this n layer 150, it is made more difficult and less likely that holes flow into the p layer 120 by the MOS gate, the holes being injected from the p layer 100 by electrons which have flown into the n− layer 110, and it makes the carrier concentration inside the n− layer 110 higher. As a result, the n− layer 110 becomes highly conductive, which enables implementation of a low-loss IGBT. Here, the formation of the n layer 150 increases the gate's feedback capacity which becomes a cause for malfunction due to noise. Accordingly, the feedback capacity is reduced by thickening the gate insulating film 300 partially.
Moreover, FIG. 12 illustrates a trench-type highly-conductive IGBT disclosed in JP-A-2000-307116. In this IGBT, a plurality of trench-gate structures T, which include a gate electrode G insulated with a gate insulating film 300, are formed on the side of an emitter electrode E alternately with two different spacings placed therebetween. Among the spacings between the trench gates, in the narrow-width portion, an n layer 151 which is in contact with an n− layer 110 is formed. A p layer 120 is formed such that it is made adjacent to this n layer 151. Also, a p+ layer 121 and an n+ layer 130, which are in low-resistance contact with the emitter electrode 600, are formed inside the p layer 120.
Meanwhile, among the spacings between the trench gates, in the broad-width portion, a p layer 125 is formed. The p layer 125 is insulated from the emitter electrode E with insulating films 401 and 402. The n layer 151 becomes a barrier against holes which are injected from the p layer 100. Accordingly, the n layer 151 exhibits an effect of accumulating electric charges within the n− layer 110, thereby enhancing the conductivity. Also, the p layer 125 has a function of collecting the holes injected from the p layer 100 into the p layer 125. These holes flow in proximity to the trench gate, then flowing into the emitter electrode E via the n layer 151, the p layer 120, and the p+ layer 121. A potential difference when the holes flow in proximity to the trench gate induces electron injection from an inversion layer of the trench gate, and further, promotes conductivity modulation of the n− layer 110. As a result of this, the IGBT becomes a low-loss IGBT.